1. Field of the Invention
This invention relates generally to heat conductive packaging of air-bridge and other low dielectric constant semiconductor chips. The invention relates more specifically to heat conductive packaging that employs an internal source of hydrogen, and methods of providing hydrogen therefor.
2. Description of the Related Art
As the minimum feature size achievable in semiconductor manufacturing continues to decrease, capacitive coupling between adjacent devices becomes a significant impediment to increased performance. Unfortunately, there are only a limited number of potential solutions to this problem. As the minimum feature size decreases, the number of devices potentially achievable in a given area increases with the inverse square of the feature size, while the space between devices decreases linearly. As the areal density of devices is raised, the amount of interconnection metallurgy must also be raised. This raises capacitive interactions between circuits on the chip, which is undesirable. Designers and process engineers, therefore, have been seeking ways to counteract this wiring capacitance problem. While various solutions have been proposed, a problem associated with several of the proposed approaches is that the heat transfer capability of the system is significantly reduced when a low dielectric constant material is employed.
For example, one approach to addressing the capacitive coupling problem has been to substitute lower dielectric constant materials for the SiO2 films (which have a dielectric constant K value of approximately 4) that are used in most VLSI chips. For example, polyimide (K equal to approximately 3.5) has been used in one commercial product where it provided a limited reduction in fringing capacitance.
It has also been proposed to electrically insulate active devices using air-gaps (i.e., “air-bridges,” with a value of K equal to 1). Such air-bridges have been limitedly employed for specialized applications. However, air-gap insulators introduce other problems. For example, they do not protect the metallurgical interconnection structures from environmental corrosion. Another major drawback associated with air insulators is that they significantly reduce the heat transfer capability of the system.
Other proposed approaches to addressing the capacitive coupling problem include filling the spaces between metal lines with carbon dioxide. “The New Low-K Candidate: It's a Gas,” Semiconductor International (March 1999), p. 38. The proposed process sequence involves a damascene process in which trenches are formed in an amorphous carbon layer followed by metal deposition to fill the trenches. After chemical mechanical polishing, a thin insulator is deposited to form a bridge layer over the metal lines. This insulator is permeable to oxygen and by implication to carbon dioxide to some extent. Presumably this process is repeated to form a multilevel metal conductor line structure. Exposure to oxygen at an elevated temperature allows oxygen to permeate into the structure and form volatile oxides of carbon. Clearly most of the gaseous products must diffuse out through the insulator in order to prevent a pressure buildup that could delaminate the insulator. When completed, the structure is free of carbon and filled with the desired CO2 insulating layers having an unspecified pressure. Unfortunately, the CO2 insulator has a thermal conductivity that is only approximately 1.5% of that of SiO2. Accordingly, chips with this insulation alone would be expected to operate at significantly higher temperatures than those made with SiO2. Some heat can still be removed through the package around the base of the silicon chip. Nevertheless, additional cooling is still required.
Even though the aforementioned solutions to the capacitive coupling problem have been proposed, providing for adequate heat removal presents a serious challenge, especially as device size is continually reduced. This evolution requires reducing conductor cross-sections, which increases electrical resistance (per unit length of conductor), thus raising resistive heating. Replacement of the traditional aluminum and aluminum alloy conductors with more conductive copper only partially reduces this mode of heat generation.
In one approach to providing enhanced heat conductivity, helium has been enveloped in the package at very modest pressures (1.6 MPa, or 1.6 atmospheres). “Thermal Conduction Module: A High-Performance Multilayer Ceramic Package,” A. J. Blodgett and D. R. Barbour, IBM Journal of Research and Development, Vol. 26, No. 1 (January 1982), pp. 30-36. The use of the slightly elevated package pressure is necessary to compensate for helium loss by permeation outward through the packaging during its lifetime.
Therefore, although different approaches have been taken to address the problem of capacitive coupling, many of the approaches suffer from the same resultant problem—a significant reduction in the heat transfer capability of the system. While the use of a helium-filled package has been proposed as a way to lessen the effect of diminished heat transfer, helium loss from permeation will occur over time, thereby lessening the heat transfer capability of the package.
Thus, no single solution to the problem of capacitive coupling has emerged that satisfies all of the technical requirements. More specifically, a need exists for a solution which not only lessens the effect of capacitive coupling, but which does so without adversely impacting the heat transfer capability of the package.